The present invention relates generally to electronic filters, and more particularly, to microstrip filters that operate at microwave and radio frequency frequencies.
Wireless communications applications have increased to crowd the available spectrum and drive the need for high isolation between adjacent bands. Portability requirements of mobile communications additionally require a reduction in the size of communications equipment. Filters used in communications devices have been required to provide improved performance using smaller sized components. Efforts have been made to develop new types of resonators, new coupling structures, and new configurations to address these requirements.
Combline filters are attractive for use in electronic communications devices. It is well known that combline filters, in general, have a natural transmission zero above its passband. One of the techniques used to reduce the number of resonators is to add cross couplings between non-adjacent resonators to provide transmission zeros. An example of this approach is shown in U.S. Pat. No. 5,543,764. As a result of these transmission zeros, filter selectivity is improved. However, in order to achieve these transmission zeros, certain coupling patterns have to be followed. This turns out to diminish the size reduction effort. In filters for wireless mobile and portable communication applications, small size and coupling structure design requirements mean that adding cross coupling to achieve transmission zeros is not a good option.
Electrically tunable microwave filters have many applications in microwave systems. These applications include local multipoint distribution service (LMDS), personal communication systems (PCS), frequency hopping radio, satellite communications, and radar systems. There are three main kinds of microwave tunable filters, mechanically, magnetically, and electrically tunable filters. Mechanically tunable filters suffer from slow tuning speed and large size. A typical magnetically tunable filter is the YIG (Yttrium-Iron-Garnet) filter, which is perhaps the most popular tunable microwave filter, because of its multioctave tuning range, and high selectivity. However, YIG filters have low tuning speed, complex structure, and complex control circuits, and are expensive.
One electronically tunable filter is the diode varactor-tuned filter, which has a high tuning speed, a simple structure, a simple control circuit, and low cost. Since the diode varactor is basically a semiconductor diode, diode varactor-tuned filters can be used in monolithic microwave integrated circuits (MMIC) or microwave integrated circuits. The performance of varactors is defined by the capacitance ratio, Cmax/Cmin, frequency range, and figure of merit, or Q factor at the specified frequency range. The Q factors for semiconductor varactors for frequencies up to 2 GHz are usually very good. However, at frequencies above 2 GHz, the Q factors of these varactors degrade rapidly.
Electronically tunable filters have been proposed that use electronically tunable varactors in combination with the filter""s resonators. When the varactor capacitance is changed, the resonator resonant frequency changes, which results in a change in the filter frequency. Electronically tunable filters have the advantages of small size, lightweight, low power consumption, simple control circuits, and fast tuning capability. Electronically tunable filters have used semiconductor diodes as the tunable capacitance. Compared with semiconductor diode varactors, tunable dielectric varactors have the advantages of lower loss, higher power handling, higher IP3, and faster tuning speed.
Commonly owned U.S. patent application Ser. No. 09/419,126, filed Oct. 15, 1999, and titled xe2x80x9cVoltage Tunable Varactors And Tunable Devices Including Such Varactorsxe2x80x9d, discloses voltage tunable dielectric varactors that operate at room temperature and various devices that include such varactors, and is hereby incorporated by reference.
Commonly owned U.S. patent application Ser. No. 09/734,969, filed Dec. 12, 2000, and titled xe2x80x9cElectronic Tunable Filters With Dielectric Varactorsxe2x80x9d, discloses microstrip filters including voltage tunable dielectric varactors that operate at room temperature, and is hereby incorporated by reference.
For miniaturization, hairpin resonator structures have been widely used in microstrip line filters, especially for high temperature superconductors (HTS). It has been noticed that a transmission zero at the low frequency side is found, which results in the filter selectivity at the low frequency side to be improved and at the high frequency side to be degraded, even though, theoretical analysis shows that the transmission zero should be at the high frequency side.
It would be desirable to provide a microstrip line filter that includes transmission zeros, but does not require cross coupling between non-adjacent resonators.
The electronic filters of this invention include a substrate, a ground conductor, a plurality of linear microstrips positioned on a the substrate with each having a first end connected to the ground conductor. A capacitor is connected between a second end of the each of the linear microstrips and the ground conductor. A U-shaped microstrip is positioned adjacent the linear microstrips, with the U-shaped microstrip including first and second extensions positioned parallel to the linear microstrips. Additional capacitors are connected between a first end of the first extension of the U-shaped microstrip and the ground conductor, and between a first end of the second extension of the U-shaped microstrip and the ground conductor. Additional U-shaped microstrips can be included. An input can coupled to one of the linear microstrips or to one of the extensions of the U-shaped microstrips. An output can be coupled to another one of the linear microstrips or to another extension of one of the U-shaped microstrips. The capacitors can be fixed or tunable capacitors. Fixed capacitors would be used to construct filters having a fixed frequency response. Tunable capacitors would be used to construct filters having a tunable frequency response. The tunable capacitors can be voltage tunable dielectric varactors.
This invention provides electronic filters including a substrate, a ground conductor, a first linear microstrip positioned on a first surface of the substrate and having a first end connected to the ground conductor, a first capacitor connected between a second end of the first linear microstrip and the ground conductor, a second linear microstrip, positioned on the first surface of the substrate parallel to the first linear microstrip, and having a first end connected to the ground conductor, a second capacitor connected between a second end of the second linear microstrip and the ground conductor, a third linear microstrip positioned on the first surface of the substrate between the first and second linear microstrips and parallel to the first and second linear microstrips, and having a first end connected to the ground conductor, a third capacitor connected between a second end of the third linear microstrip and the ground conductor, a U-shaped microstrip positioned between the first and third linear microstrips, the U-shaped microstrip including first and second extensions positioned parallel to the first, second and third linear microstrips, a fourth capacitor connected between a first end of the first extension of the U-shaped microstrip and the ground conductor, a fifth capacitor connected between a first end of the second extension of the U-shaped microstrip and the ground conductor, an input coupled to the first linear microstrip, and an output coupled to the second linear microstrip.
The invention also encompasses electronic filters including a substrate, a ground conductor, a first linear microstrip positioned on a first surface of the substrate and having a first end connected to the ground conductor, a first capacitor connected between a second end of the first linear microstrip and the ground conductor, a second linear microstrip, positioned on the first surface of the substrate parallel to the first linear microstrip, and having a first end connected to the ground conductor, a second capacitor connected between a second end of the second linear microstrip and the ground conductor, a first U-shaped microstrip positioned between the first and second linear microstrips, the first U-shaped microstrip including first and second extensions positioned parallel to the first and second linear microstrips, a third capacitor connected between a first end of the first extension of the first U-shaped microstrip and the ground conductor, a fourth capacitor connected between a first end of the second extension of the first U-shaped microstrip and the ground conductor, a second U-shaped microstrip positioned between the first and second linear microstrips, the second U-shaped microstrip including third and fourth extensions positioned parallel to the first and second linear microstrips, a fifth capacitor connected between a first end of the third extension of the second U-shaped microstrip and the ground conductor, a sixth capacitor connected between a first end of the fourth extension of the second U-shaped microstrip and the ground conductor, an input coupled to the first linear microstrip, and an output coupled to the second linear microstrip.
The invention further encompasses electronic filters including a substrate, a ground conductor, a first linear microstrip positioned on a first surface of the substrate and having a first end connected to the ground conductor, a first capacitor connected between a second end of the first linear microstrip and the ground conductor, a second linear microstrip, positioned on the first surface of the substrate parallel to the first linear microstrip, and having a first end connected to the ground conductor, a second capacitor connected between a second end of the second linear microstrip and the ground conductor, a first U-shaped microstrip positioned between the first and second linear microstrips, the first U-shaped microstrip including first and second extensions positioned parallel to the first and second linear microstrips, a third capacitor connected between a first end of the first extension of the first U-shaped microstrip and the ground conductor, a fourth capacitor connected between a first end of the second extension of the first U-shaped microstrip and the ground conductor, a second U-shaped microstrip positioned between the first and second linear microstrips, the second U-shaped microstrip including third and fourth extensions positioned parallel to the first and second linear microstrips, a fifth capacitor connected between a first end of the third extension of the second U-shaped microstrip and the ground conductor, a sixth capacitor connected between a first end of the fourth extension of the second U-shaped microstrip and the ground conductor, an input coupled to the first extension of the first U-shaped microstrip, and an output coupled to the fourth extension of the second U-shaped microstrip.
The filters of this invention can utilize combinations of combline and hairpin resonators to provide transmission zeros at both the upper and lower sides of the filter passband. Tunable versions of the filters provide consistent bandwidth and insertion loss in the tuning range.